Low Alloy X100 Pipeline Steel Corrosion and Passivation Behavior in Bicarbonate-Based Solutions of pH 6.7 to 8.9 with Groundwater Anions: An Electrochemical Study

Abstract

This research investigates the fundamental corrosion and passivation processes occurring on API-X100 pipeline steels before, during, and after passive layer formation in bicarbonate-based simulated soil solutions of pH 6.7 to 8.9. Free corrosion potentials decrease exponentially with bicarbonate (and pH), owing to increased water and bicarbonate reduction in more alkaline conditions and the coupled iron oxidation reaction. Active corrosion rates at potentials slightly above open circuit potential increase with bicarbonate, until a concentration of 1.68 g L À1 sodium bicarbonate (plus dilute amounts of chlorides/sulfate) at which first signs of film formation appear. Thereon, increased bicarbonate concentration generally decreases current densities and resists ferrous-oxide product formations due to improved iron carbonate formation conditions precipitating more durable passive layers. Potentiodynamic polarization in the anodic regime reveals varying electrochemical processes involving interactions between hydroxide, carbonate, and oxide complexes with ferrous, with diverse pH and potential dependencies. The products of corrosion reactions were investigated by scanning electron microscopy, X-ray diffraction, and/or X-ray photoelectron spectroscopy.

A study of X100 pipeline steel passivation in mildly alkaline bicarbonate solutions using electrochemical impedance spectroscopy under potentiodynamic conditions and Mott–Schottky

“…These results, in addition to the detection of iron hydroxide complexes in the ex-situ X-ray diffraction (XRD) characterization of low alloy steel corrosion products (multi-layered) in similar environments [28], support the notion that the transitory current density behavior directly above −0.5 V SCE involves Fe(OH) 2 formation and dissolution. Since the potential range of this region is very small, it is difficult to target the specific regions of decreasing or increasing current density with a potentiostatic polarization.…”

“…Values of E p1 in mildly alkaline bicarbonate media are within the E-pH region of Fe(OH) 2 dominance in the Fe-H-C-O Pourbaix, as shown in [28] based on previous work studying the anodic hydrogenation of iron in HCO 3 − /CO 3 2− solutions [26]. These results, in addition to the detection of iron hydroxide complexes in the ex-situ X-ray diffraction (XRD) characterization of low alloy steel corrosion products (multi-layered) in similar environments [28], support the notion that the transitory current density behavior directly above −0.5 V SCE involves Fe(OH) 2 formation and dissolution.…”

Electrochemical and Passive Layer Characterizations of 304L, 316L, and Duplex 2205 Stainless Steels in Thiosulfate Gold Leaching Solutions

“…This same interpretation of OCP behavior applies to steels in other systems, for example, as previously shown on low-alloy steel in a variety of groundwater environments. 55 Since the concentrations of dissolved oxygen and iron cations are expected to remain relatively constant during the immersion, the surface state of the electrode, specifically the presence of a passive oxide film, is the main influence on the rate of cathodic and anodic reactions. The stabilized OCP values after 120 h of immersion for all three alloys correspond to the passive-like region of the anodic branches in Figure 2, verifying the presence of a passive oxide film on all alloys.…”